Worm support device and power assist unit having the same

ABSTRACT

Deep groove ball bearings are used as bearings supporting both side portions of a worm. In the first bearing on the side of a motor, the curvature radius of a raceway groove in an inner ring or the curvature radius of a raceway groove in an outer ring are set to a specific numerical range with respect to the diameter of a ball.

BACKGROUND OF THE INVENTION

The present invention relates to a worm support device and a powerassist unit having the same.

Electric power steering apparatuses for automobiles are equipped with apower assist unit providing a steering assist power to a wheel steeringmechanism. The power assist unit comprises a motor and a worm gearmechanism. The worm mechanism includes a worm shaft and a worm wheel. Anappropriate backlash is provided in the engagement portion of the wormand worm wheel. Because of the backlash, when the worm is rotated inreverse, the tooth surface of the worm collides with the tooth surfaceof the worm wheel, producing a knocking sound. In order to overcome thisdrawback, an elastic body such as a spring or an O ring is insertedbetween the bearing for supporting the worm and the housing or worm(Japanese Patent Applications Laid-open No. H11-43062 and H11-171027).

In the above-described conventional example, an elastic body such as aspring or an O ring is required and the number of parts and assemblingoperations is increased. Further, changes in the elastic constant of theelastic body with time also cause a concern.

BRIEF SUMMARY OF THE INVENTION

The worm support device in accordance with the present invention is aworm support device for supporting shaft sections on both end sides of aworm connected to a drive source on a housing via bearings, wherein adeep groove ball bearing satisfying at least either a first conditionwhere the curvature radius of a racetrack groove of an inner ring isbetween 52.5% or more and 75% or less of the diameter of a ball or asecond condition where the curvature radius of a racetrack groove of anouter ring is between 53.5% or more and 85% or less of the diameter of aball is used for the bearing (first bearing) on the drive source side.

In sum, the first bearing is a deep groove ball bearing and the elasticconstant in the axial direction thereof is appropriately decreased. Inother words, when a rotary drive force acts upon the shaft section ofthe worm, the balls of the first bearing roll in the axial direction,while elastically bending and deforming a racetrack groove of the innerring and/or the racetrack groove of the outer ring. Therefore, the innerring and outer ring shift in the axial direction and allow for axialdisplacement of the shaft sections of the worm. Because the axialdisplacement of the shaft sections of the worm is thus allowed by usingthe elasticity of the inner ring or outer ring, the shaft sections ofthe worm move gradually in a state with the prescribed tension and areprevented from moving rapidly. As a result, for example, when the wormwheel is engaged with the gear section of the worm, the knocking soundproduced when the tooth surface of the gear section is collided with thetooth surface of the worm wheel is reduced or prevented.

The inner ring of the first bearing is tightly fit with respect to theshaft section of the worm, and the bearing (second bearing) supportingthe side of the haft section opposite to the drive source can be sojoined that it can move with respect to the shaft section of the worm orthe housing. The second bearing may have the same configuration as thefirst bearing and may be a rolling bearing such as a needle rollingbearing or a sliding bearing such as a bushing.

In this case, as described hereinabove, the worm shaft can move smoothlywhen it is displaced in the axial direction.

Further, the first bearing can be set to a negative gap. In this case,as described above, when the balls of the first bearing and the innerand outer rings move in the axial direction, the play thereof isprevented.

A power assist unit in accordance with the present invention comprises amotor and a worm gear mechanism for reducing the rotational motiongenerated by the motor and outputting it as the steering assist power,wherein the worm gear mechanism comprises a worm coupled to the outputshaft of the motor and a worm wheel engaged with the gear section of theworm and externally fixed to the rotary shaft, a first bearing on themotor side and a second bearing on the side opposite the motor side forsupporting the respective shaft sections on both end sides of the wormon the housing, and the housing for accommodating at least the worm andboth bearings in a supported state thereof, and a deep-groove ballbearing satisfying at least either a first condition where the curvatureradius of a racetrack groove of an inner ring is between 52.5% or moreand 75% or less of the diameter of a ball or a second condition wherethe curvature radius of a racetrack groove of an outer ring is between53.5% or more and 85% or less of the diameter of a ball is used for thefirst bearing.

The configuration of this power assist unit is similar to that of theabove-described worm support device. Therefore, the knocking soundproduced when the tooth surface of the gear section is collided with thetooth surface of the worm wheel is reduced or prevented.

Moreover, in the case of the conventional power assist unit, the rotarydrive force acts upon the worm from the worm wheel within the intervalfrom the steering of the steering wheel to the action of the steeringassist power from the motor upon the worm (called the initial stage ofsteering). Because the motor has a large inertial weight, however, thesteering feeling of the driver is unfavorable, for example, the steeringcannot be conducted since the steering feel is heavy.

With this respect, too, with the above-described configuration inaccordance with the present invention, if the bearing for worm supportis a deep groove ball bearing and the elastic constant in the axialdirection thereof is set appropriately low, then when a rotary driveforce acts from the worm wheel upon the worm in the initial stage ofsteering, the worm is gradually displaced in a state with the prescribedtension in the axial direction before the worm is rotated due to elasticproperties of the bearing. Therefore, no feeling of discomfort isinduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Accompanying the specification are figures which assist in illustratingthe embodiments of the invention, in which:

FIG. 1 is a side view illustrating an electric power steering device ofthe preferred mode for carrying out the invention;

FIG. 2 is an enlarged partial cross-sectional view of the power assistunit, as viewed from the section along the (2)-(2) line in FIG. 1;

FIG. 3 is a cross-sectional view showing an enlarged upper half of thefirst bearing shown in FIG. 2;

FIG. 4 is a cross-sectional view illustrating another example of thefirst bearing shown in FIG. 3;

FIG. 5 is a cross-sectional view illustrating yet another example of thefirst bearing shown in FIG. 3;

FIG. 6 is an enlarged main cross-sectional view of another power assistunit;

FIG. 7 illustrates the configuration of another electric power steeringdevice;

FIG. 8 is a cross-sectional view illustrating an enlarged upper half ofanother bearing;

FIG. 9 shows the relationship between the shift of the worm in the axialdirection and the load applied to the racetrack grooves;

FIG. 10 is an enlarged main cross-sectional view of another power assistunit;

FIG. 11 is an enlarged main cross-sectional view of yet another powerassist unit;

FIG. 12 is an enlarged main cross-sectional view of yet another powerassist unit; and

FIG. 13 is an enlarged main cross-sectional view of yet another powerassist unit.

DETAILED DESCRIPTION OF THE INVENTION

One preferred embodiment for carrying out the invention is illustratedby FIGS. 1 to 3. The electric power steering apparatus 1 of the exampleshown in the figures comprises a steering shaft 2 and a power assistunit 3.

The steering shaft 2 transmits a steering force to a wheel steeringmechanism (not shown) in response to the rotation control of thesteering wheel (not shown) and comprises an input shaft 5 having thesteering wheel mounted thereon and an output shaft 6 linked to the wheelsteering mechanism, the two shafts being joined with a torsion bar (notshown).

The power assist unit 3 applies a rotational motion to the steeringshaft 2 when a steering force (rotary drive force) was inputted to thesteering shaft 2 following the rotary control of the steering wheel andcomprises a motor 10 and a worm gear mechanism 11.

The motor 10 outputs a rotary power in response to a command from acontrol unit (not shown). The worm gear mechanism 11 reduces thesteering assisting force outputted from the motor 10 and outputs thesteering assisting force relating to the steering shaft 2. The worm gearmechanism 11 comprises a worm wheel 13 and a worm 14.

The worm wheel 13 is externally fixed to the steering shaft 2 on theside of the output shaft 6. The worm 14 has a worm gear section 15 a(FIG. 2) formed in the intermediate portion thereof in the axialdirection, and this worm gear section 15 a is engaged with the wormwheel 13. The worm 14 is coupled to the output shaft 10 a of the motor10 via a coupling 16. Respective spline teeth are formed on the outerperiphery of the worm 14 on the side of the motor 10, inner periphery ofthe coupling 16, and outer periphery of the output shaft 10 a of themotor 10. Those three groups are spline mated, thereby providing forintegration in the rotation direction, while being free to move withrespect to each other in the axial direction. In the worm 14, worm shaftsections 15 b, 15 c on both ends of the worm gear section 15 a formed inthe intermediate portion of the worm in the axial direction arerotatably supported by respective bearings 20, 30 with respect to thehousing 17.

Turning to FIG. 3, the two bearings 20, 30 are deep groove ballbearings. Among them, the first bearing 20 on the side of the motor 10comprises an inner ring 21, an outer ring 22, a plurality of balls 23,and a retainer 24. The inner space of the first bearing 20 is sealedwith a seal 25, and a lubricant such as grease is introduced into theinner space of the bearing. The seal 25 is of a contactless type calleda sealed plate, the outer peripheral portion thereof is mounted on bothends of the outer ring 22 in the axial direction, and the innerperipheral portion produces a contactless sealing portion facing theinner ring 21 via a tiny gap.

Similarly to the first bearing 20, the second bearing 30 disposed on theend side in the worm 14 comprises an inner ring 31, an outer ring 32, aplurality of balls 33, and a retainer 34 (this configuration is notshown in detail). The outer diameter size of the first bearing 20 is setlarger than that of the second bearing 30. A seal for sealing the insideof the bearings may be used, but the use thereof is optional.

The configuration of the first and second bearings 20, 30 and the modeof attaching the first and second bearings 20, 30 to the worm 14 will bedescribed below.

In the first bearing 20 disposed on the side of the motor 10, as shownin FIG. 3, the curvature radius R1 of the racetrack groove 21 a of theinner ring 21 is set at from 52.5% or more to 75% or less, preferablyfrom 52.5% or more to 70% or less, even more preferably from 52.5% ormore to 65% or less of the radius r of the balls 23.

The curvature radius R2 of the racetrack groove 22 a of the outer ring22 is set at from 53.5% or more to 85% or less, preferably from 53.5% ormore to 80% or less, even more preferably from 53.5% or more to 75% orless of the radius r of the balls 23. In the first bearing 20, anegative gap is set, that is, the radial inner gap is set as a negativevalue.

The “%” above represents the curvatures M1, M2 of the racetrack grooves21 a, 22 a, respectively, which are given by (r/R1)×100% and(r/R2)×100%. In the present embodiment the curvature M1 is set to 60%and the curvature M2 is set to 70%. As the upper limit of the ranges ofthe curvatures M1, M2 decreases, the elastic constant of the bearings20, 22 decreases and a larger axial displacement of the worm 14 isallowed. Therefore, in order to guarantee the margin of the axialdisplacement of the worm 14, the maximum upper limit for the range ofthe curvatures M1, M2 is set at 75% for the curvature M1 and 85% for thecurvature M2, and various settings below those upper limits arepossible.

On the other hand, if the lower limit of the ranges of the curvaturesM1, M2 is too low, the above-mentioned elastic constant becomes toosmall and the axial displacement of the worm 14 becomes too larger. Forthis reason, the lower limits of the curvatures M1, M2 are set to theabove-described constant values, that is, 52.5% for the curvature M1 and53.5% for the curvature M2.

In the second bearing 30, the curvature radius R3 of the racetrackgroove 31 a of the inner ring 31 is set at from 51.5% or more to 52.5%or less, preferably from 51.5% or more to 52% or less, of the radius rof the balls 33. Further, the curvature radius R4 of the racetrackgroove 32 a of the outer ring 32 is set at from 52.5% or more to 53% orless of the radius r of the balls 33. In the second bearing 30, anegative gap is set, that is, the radial inner gap is set as a negativevalue. The “%” above represents the curvatures M3, M4 of the racetrackgrooves 31 a, 32 a, respectively, which are given by (r/R3)×100% and(r/R4)×100%.

In the first and second bearings 20, 30, the hardness of the balls 23,33 is set higher than the hardness of the inner rings 21, 31 and outerrings 22, 32, similarly to the generally used products.

The joining state of the inner ring 21 (FIG. 2) of the first bearing 20and the worm shaft section 15 b on one end side of the worm gear section15 a in the worm 14 is set to “tight fitting”. On the other hand, thejoining state of the inner ring 31 of the second bearing 30 disposed onthe opposite side of the motor 10 and the worm shaft section 15 c on theother end side of the worm gear section 15 a of the worm 14 is set to“loose fitting”.

The outer ring 22 of the first bearing 20 is “loose fitted” with respectto a large-diameter inner peripheral surface 17 b of the innerperipheral surfaces of the housing 17 and positioned in the axialdirection by sandwiching from the axial direction between a step wallsurface 17 a obtained due to the step between a small-diameter innerperipheral surface 17 c and the large-diameter inner peripheral surface17 b of the housing 17 and a threaded lid which is mounted by threadingon the large-diameter inner peripheral surface 17 b of the housing 17.On the other hand, the outer ring 32 of the second bearing 30 is joinedby press fitting into the small-diameter inner peripheral surface 17 cof the housing 17 and positioned in the axial direction.

If the curvature radii R1, R2 of the first bearing 20 are thus set ashigh as possible, the elastic constant of the first bearing 20 in theradial direction decreases appropriately. In other words, when the innerring 21 that is externally fit on the worm 14 and the external ring 22that is internally fit in the housing 17 are impelled toward each otherin the axial direction by the application of an axial load to the worm14, the balls 23 of the first bearing 20 roll in the axial direction,while causing elastic bending of the racetrack groove 21 a of the innerring 21 and the racetrack groove 22 a of the outer ring 22. Therefore,the inner ring 21 and outer ring 22 are shifted in the axial directionallowing for axial displacement of the worm 14. As described above,because the inner ring 31 of the second bearing 30 is loosely fit withrespect to the worm 14, a smooth movement is executed when the worm 14is displaced, as described above, in the axial direction.

Moreover, because a pressure is applied upon setting the first bearing20 to a negative gap, no play occurs when the balls 23 and inner andouter rings 21, 22 move with respect to each other in the axialdirection, as described hereinabove.

Thus, because the axial displacement of the worm 14 is allowed by usingthe elasticity of the inner ring and outer ring 21, 22, the worm 14moves gradually in a state with the prescribed tension and is preventedfrom moving rapidly.

More specifically, within the interval from steering the steering wheelto the action of the steering assist power from the motor 10 upon theworm 14 (called the initial stage of steering) or during the reversesteering of the steering wheel, the worm 14 is gradually displaced inthe axial direction in a state with the prescribed tension by theelasticity of the first bearing 20 and the worm 14 is prevented fromdisplacing rapidly. As a result, the steering feel within the intervalprior to the steering assist power application from the motor 10 can beimproved, for example, no feeling of discomfort is induced. Further,during reverse steering of the steering wheel, the knocking soundproduced when the tooth surface of the worm gear section 15 a of theworm 14 collides with the tooth surface of the worm wheel 13 is reducedor prevented. The inner ring 31 of the second bearing 30 may be fittightly and the outer ring 32 may be set to a loose fitting.

In the above-described configuration, because no extra parts such assprings were used, the number of parts can be decreased and changes inthe elastic constant caused by deterioration with time can be avoided.

The racetrack grooves 21 a, 22 a in the inner and outer rings 21, 22 ofthe first bearing 20 can be the composite curved surfaces, as shown inFIGS. 4 and 5. In FIG. 4, the racetrack grooves 21 a, 22 a are formedfrom two curved surfaces. The curvature radius R11 in both shoulderregions X2 a, X2 b is set smaller than the curvature radius R10 in thegroove bottom region X1. In FIG. 5, the racetrack grooves 21 a, 22 a areformed from three curved surfaces. The relationship R10>R12>R11 is setbetween the curvature radius R10 in the groove bottom region X1,curvature radius R12 in the intermediate regions X3 a, X3 b, andcurvature radius R11 in the two shoulder regions X2 a, X2 b. When suchcomposite curved surfaces are produced, the balls 23 are reliablyprevented from rolling past the two shoulder regions X2 a, X2 b whenthey roll in the axial direction. Therefore, it is possible to specify arange in which the inner and outer rings 21, 22 can displace withrespect to each other in the axial direction.

In the above-described embodiment, the curvature radii of the racetrackgrooves in the inner and outer rings 31, 32 of the second bearing 30disposed on the distal end side of the worm 14 can be set similarly tothose of the first bearing 20 explained hereinabove in the embodiment.Furthermore, the curvature radius of the racetrack groove in any one ofthe inner rings 21, 31 and outer rings 22, 32 may be also set within anumerical range set forth by the JIS standard.

The above-described embodiment relates to the electric power steeringapparatus of a column assist type having a power assist unit 3 (FIG. 1)disposed on a steering shaft 2. However, an electric power steeringapparatus can be also of a pinion assist type where the power assistunit 3 is disposed in a steering gear box. In this case, the worm wheel11 of the power assist unit 3 is externally mounted on the pinion shaftof the steering gear box.

As described hereinabove, in the worm support apparatus in accordancewith the present invention, the axial displacement of the worm isallowed by using the elasticity of the inner ring or outer ring of thefirst bearing. Therefore, when a rotary drive force acts upon the worm,the worm can move slowly in a state with the prescribed tension and canbe prevented from moving rapidly. Therefore, a knocking sound producedwhen the worm wheel engaged with the gear section of the worm collidestherewith can be reduced or prevented.

Further, in the power assist unit in accordance with the presentinvention, in the same manner as described hereinabove, when the toothsurface of the gear section of the worm collides with the tooth surfaceof the worm wheel, the knocking sound can be reduced or prevented.Moreover, when a rotary drive force acts from the worm wheel upon theworm in the initial stage of steering of the steering wheel, the wormcan be displaced gradually in the axial direction in a state with theprescribed tension, thereby providing for improvements, such as lack ofdiscomfort during steering.

Another mode of the present invention will be described below.

Referring to the power assist unit shown in FIG. 6 and an electric powersteering device (shown in FIG. 7) incorporating the power assist unitshown in FIG. 6, this electric power steering device comprises a motor101 for steering assist and a worm gear mechanism 100A. The mechanism100A has a worm 103 as a small gear connected to the drive shaft 101 aof the motor 101 via a coupling 102 having a male coupling 102 a and afemale coupling 102 b and a worm wheel 104 as a large gear engaged withthe worm 103. The device has a housing 105 serving as a support memberfor accommodating and supporting the worm gear mechanism 100A, andsteering means 106 connected to the worm gear mechanism 100A.

The steering means 106 comprises an input shaft 161 connected by one endthereof to the steering wheel 100B for steering and having a tubularsection 161 a at the other end, a torsion bar 162 inserted into thetubular section 161 a, coupled by one end thereof to the tubular section161 a of the input shaft 161, and twisted by the action of the steeringtorque applied to the steering wheel 100B, and an output shaft 163coupled by the other end thereof to the other end section of the torsionbar 162 and connected to the worm gear mechanism 100A, wherein theoutput shaft 163 is connected to a steering mechanism (not shown), forexample, of a rack-and-pinion system, via a universal joint.

The housing 105 has a first accommodation section 105 a thataccommodates the worm 103 having shaft sections 103 b, 103 c at bothends of the tooth section 103 a and rotatably supports the shaftsections 103 b, 103 c of the worm 103 via the deep groove bearings 107,108 and a second accommodation section 105 b that accommodates the wormwheel 104 and supports the worm wheel 104 via an output shaft 163 andtwo deep groove bearings 109, 110 mated with the output shaft 163.

The first accommodation section 105 a extends in the axial direction ofthe worm 103, and a support hole 151 for mating with and supporting theouter ring 107 a of the bearing 107, a motor mounting section 153 and athreaded hole 152 connected to one end of the support hole 151, and acontrol section 154 (e.g., a step in housing structure) connected to theother end of the support hole 151 and serving to control the movement ofthe bearing 107 are provided in one end section of the firstaccommodation section in the longitudinal direction thereof.

Further, the outer ring 107 a of the bearing 107 is joined to thesupport hole 151, an annular threaded lid 111 abutting against one endof the outer ring 107 a is threaded into the threaded hole 152, and theother end section of the outer ring 107 a is pressed against the controlsection 154. Further, the motor 101 is mounted on the motor mountingsection 153.

A support hole 155 for mating with and supporting the outer ring 108 aof the bearing 108 and a control section 156 connected to one end of thesupport hole 155 and serving to control the movement of the outer ring108 a of the bearing 108 are provided in the other end section of thefirst accommodation section 105 a. The other end section of the supporthole 155 is opened to the outside, a lid 113 is threaded into the openportion, and the other end of the outer ring 108 a is pressed againstthe control section 156.

In the worm 103 of the worm gear mechanism 100A, the shaft section 103 bprovided at one end of the tooth section 103 a having a plurality ofteeth is inserted, so that it is free to move in the axial direction,into the inner ring 107 b of the bearing 107 and rotatably supported inthe support hole 151 via the bearing 107. The shaft section 103 cprovided at the other end of the tooth section 103 a is inserted, sothat it is free to move in the axial direction, into the inner ring 108b of the bearing 108 and rotatably supported in the support hole 155 viathe bearing 108. The inner rings 107 b, 108 b abut against the tow endsof the tooth section 103 a. When the worm 103 moves in one axialdirection (rightward), the inner ring 107 b is pushed in the one axialdirection, and when the worm 103 moves in the other axial direction(leftward), the inner ring 108 b is pushed in the other axial direction.

The worm wheel 104 is mated with and fixed to the intermediate portionof the output shaft 163.

FIG. 8 is an enlarged cross-sectional view of part of the bearing.

The value of the axial inner gap of the bearings 107, 108 that supportsuch worm 103 is larger than the standard value specified by theJapanese Industrial Standard. More specifically, if the diameter ofballs 107 c, 108 c is denoted by d, and the radius of racetrack grooves107 d, 108 d of the inner rings 107 b, 108 b is denoted by R, then whenthe curvature radius d/R of the racetrack grooves 107 d, 108 d relatingto the balls 107 c, 108 c is set to a value (for example, 60-80%) largerthan the value (53%) set forth by the Japanese Industrial Standard, thevalue of the axial inner gap will be larger than the standard value ofthe Japanese Industrial Standard and the worm 103 supported by thebearings 107, 108 can be moved in the axial direction with respect tothe outer rings 107 a, 108 a. Further, the radii of the balls 107 c, 108c and racetrack grooves 107 e, 108 e of the outer rings 107 a, 108 a arethe standard values. The relationship between the axial inner gap andradial gap is defined by the JIS.

The output shaft 101 a (FIG. 6) of the motor 101 and the shaft section103 b of the worm 103 are so joined that they can move with respect toeach other in the axial direction via a male coupling 102 a and femalecoupling 102 b having serrations. The male coupling 102 a is constitutedby providing serrations on the peripheral surface of the shaft section103, and the female coupling 102 b is constituted by providingserrations inside the tubular member 102 c mated with and fixed to thedrive shaft 101 a. The male coupling 102 a and female coupling 102 b aremated by the serrations.

A torque sensor 112 (FIG. 7) for detecting the steering torque appliedto the steering wheel 100B based on the relative rotation displacementquantity of the input shaft 161 and output shaft 163 corresponding totwisting of the torsion bar 162 is contained in the housing 105, and thedrive control is also conducted based on the torque detected with thetorque sensor 112.

FIG. 9 illustrates the relationship between the shift of the worm in theaxial direction and the load applied to the racetrack grooves 107 d, 108d. FIG. 9 shows that when the shift and load are positive, a force inone axial direction (rightward) is applied to the worm 103 and moves itin the one axial direction (rightward), and when the shift and load arenegative, a force in the other axial direction (leftward) is applied tothe worm 103 and moves it in the other axial direction (leftward).

In the electric power steering apparatus of the above-describedconfiguration, the shaft section 103 b (FIG. 6) of the worm 103 in whichthe shaft section 103 b at one end is coupled via the coupling 102 tothe drive shaft 101 a is rotatably supported by the bearing 107, theshaft section 103 c is rotatably supported by the bearing 108, and theworm 103 can be moved in the axial direction with respect to the innerrings 107 b, 108 b. Further, the balls 107 c of the bearings 107, 108are positioned in the central sections of the racetrack grooves 107 d,108 d, 107 e, 108 e of the outer rings 107 a, 108 a and inner rings 107b, 108 b. The bearings 107, 108 allow the worm 103 to be moved in theaxial direction with respect to the outer rings 107 a, 108 a because thevalue of the axial inner gap is larger than the JIS-specified value.Furthermore, the shift of the worm 103 in the axial direction can belarger than the shift (d) in the case where the conventional ballbearing with the JIS-specified value of the axial inner gap was used, asshown in FIG. 9.

Thus, when a steering force of the steering wheel 100B (FIG. 7) istransferred to the worm 103 via the input shaft 161, torsion bar 162,output shaft 163, and worm wheel 104 by steering in a steering regionwithout driving from the motor 101, that is, in a steering region wherea steering angle during high-speed movement of a vehicle is, forexample, as small as about one degree (1°), the following occurs. Theworm 103 is moved in one axial direction (rightward) with respect to theouter ring 107 a, while pushing the inner ring 107 b, or in the otheraxial direction (leftward) with respect to the outer ring 108 a, whilepushing the inner ring 108 b, by the axial force component applied tothe worm 103, the rotation angle of the worm 103 decreases, thetransmission of force from the worm 103 to the drive shaft 101 a of themotor 101 can be relaxed, the steering load in the steering regionwithout driving from the motor 104 can be reduced, and steering feelingcan be improved. Further, when the worm 103 moves in one axial direction(rightward), the shaft section 103 c and inner ring 108 b move withrespect to each other, and when the worm 103 moves in the other axialdirection (leftward), the shaft part 103 b and inner ring 107 b movewith respect to each other.

Further, because in this configuration the bearings 107, 108 forsupporting the worm 103 were improved, without adding any specialmechanism, the structure can be simplified and the worm 103 can beminiaturized, despite the fact that the steering load in a steeringregion without driving from the motor 101 can be reduced.

Referring to FIG. 10, the power assist unit shown in this figurecomprises elastic rings 114, 115 as suppression means for suppressingthe relative movement of the inner ring 107 b and outer ring 107 a andthe inner ring 108 b and outer ring 108 a of the bearings 107, 108 inthe axial direction. The worm 103 can move in the axial direction withrespect to the inner rings 107 b, 108 b.

Referring to FIG. 10, locking rings 116, 117 are provided in theintermediate portions of the shaft sections 103 b, 103 c, elastic rings114, 115 such as plate springs, elastic washers, or rubber sheets areprovided between locking rings 116, 117 and inner rings 107 b, 108 b,and the elastic rings 114, 115 displace the inner rings 107 b, 108 btoward the tooth section 103 a with respect to the outer rings 107 a,108 a and prevent the inner rings 107 b, 108 b from floating in theaxial direction with respect to the outer rings 107 a, 108 a.

When the worm 103 moves axially in one direction (rightward) in thesteering region without driving from the motor 101, the tooth section103 a of the worm 103 applies pressure to the inner ring 107 b, and theworm 103 moves rightward together with the inner ring 107 b, whilebending the elastic ring 114, and inhibits the relative movement of theinner ring 107 b and outer ring 107 a in the axial direction. Themovement quantity of the worm 103 decreases as the bending quantity ofthe elastic ring 114 increases. Furthermore, when the worm 103 movesaxially in the other direction (leftward), the tooth section 103 a ofthe worm 103 applies pressure to the inner ring 108 b and the worm 103moves leftward together with the inner ring 108 b, while bending theelastic ring 115, and inhibits the relative movement of the inner ring108 b and outer ring 108 a in the axial direction. The movement quantityof the worm 103 decreases as the bending quantity of the elastic ring115 increases. Further, when the worm 103 moves axially in one direction(rightward), the elastic ring 115 is bent via the locking ring 117, butthe inner ring 108 b does not move. When the worm 103 moves axially inthe other direction (leftward), the elastic ring 114 is bent via thelocking ring 116, but the inner ring 107 b does not move.

Further, because the relative movement of the inner rings 107 b, 108 band outer rings 107 a, 108 a in the axial direction is inhibited, whenthe worm 103 and bearings 107,108 are assembled, the position of theinner rings 107 b, 108 b in the axial direction relative to the outerrings 107 a, 108 a and the position of the worm 103 in the axialdirection relative to the bearings 107, 108 can be easily set and theassemblability can be improved.

Referring to FIG. 11, the power assist unit shown in this figurecomprises elastic rings 118, 119, 120, 121 serving as inhibition meansfor inhibiting the relative movement of the inner rings 107 b, 108 b inthe axial direction relative to outer rings 107 a, 108 a and allowingthe bearings 107, 108 to move in the axial direction with respect to theouter rings 107 a, 108 a. The worm 103 can move in the axial directionwith respect to the inner rings 107 b, 108 b.

The outer rings 107 a, 108 a are fit, so that they are free to move inthe axial direction, into support holes 151, 155, and elastic rings 120,121 such as plate springs, elastic washers, or rubber sheets areprovided between the outer rings 107 a, 108 a and control sections 154,156. Locking rings 122, 123 are provided in the intermediate portions ofthe shaft sections 103 b, 103 c, elastic rings 118, 119 such as platesprings, elastic washers, or rubber sheets are provided between thelocking rings 122, 123 and inner rings 107 b, 108 b, and the elasticrings 118, 119 displace the inner rings 107 b, 108 b toward the toothsection 103 a with respect to the outer rings 107 a, 108 a and preventthe inner rings 107 b, 108 b from floating in the axial direction withrespect to the outer rings 107 a, 108 a.

When the worm 103 moves axially in one direction (rightward) in thesteering region without driving from the motor 101, the tooth section103 a of the worm 103 applies pressure to the inner ring 107 b, and theworm 103 moves rightward together with the inner ring 107 b, whilebending the elastic ring 118, and inhibits the relative movement of theinner ring 107 b and outer ring 107 a in the axial direction. Themovement quantity of the worm 103 decreases as the bending quantity ofthe elastic ring 118 increases. Furthermore, when the worm 103 movesaxially in the other direction (leftward), the tooth section 103 a ofthe worm 103 applies pressure to the inner ring 108 b and the worm 103moves leftward together with the inner ring 108 b, while bending theelastic ring 119, and inhibits the relative movement of the inner ring108 b and outer ring 108 a in the axial direction. The movement quantityof the worm 103 decreases as the bending quantity of the elastic ring119 increases. Further, when the worm 103 moves axially in one direction(rightward), the elastic ring 119 is bent via the locking ring 123, theelastic ring 119 is also bent via the inner ring 108 b, a rolling body108 c, and the outer ring 108 a, the elastic ring 121 bends via theinner ring 108 b, rolling body 108 c, and outer ring 108, and the entirebearing 108 moves axially in the one direction (rightward). Further,when the worm 103 moves axially in the other direction (leftward), theelastic ring 118 is bent via the locking ring 122, the elastic ring 120is bent via the inner ring 107 b, a rolling body 107 c, and the outerring 107 a, and the entire bearing 107 moves axially in the otherdirection (leftward).

Further, because the relative movement of the inner rings 107 b, 108 band outer rings 107 a, 108 a in the axial direction is inhibited, whenthe worm 103 and bearings 107,108 are assembled, the position of theinner rings 107 b, 108 b in the axial direction relative to outer rings107 a, 108 a and the position of the worm 103 in the axial directionrelative to the bearings 107, 108 can be readily set and theassemblability can be improved.

Referring to FIG. 12, in the power assist unit shown in this figure, abearing member 124 for bearing the shaft section 103 c is provided toenable the adjustment of the distance between the rotation centers ofthe worm 103 and worm wheel 104 instead of the bearing 108 located onthe opposite side of the motor. Further, elastic rings 125, 126 servingas inhibition means for inhibiting the relative axial movement of theinner ring 107 b and outer ring 107 a of the bearing 107 of Embodiment1, which is disposed on the motor side, are provided on both end sidesof the inner ring 107 b, and the shaft section 103 c can move in theaxial direction with respect to the bearing member 124.

The worm 103 is specified to be able to move in the axial direction withrespect to the bearing member 124. A locking ring 127 is provided in theintermediate portion of the shaft section 103 b, elastic rings 125, 126,such as plate springs, elastic washers, or rubber sheets are providedbetween the locking ring 127 and inner ring 107 b and between the toothsection 103 a and inner ring 107 b, the elastic rings 125, 126 positionthe inner ring 107 b in the center in the axial direction of the outerring 107 a, and the inner ring 107 b is prevented from floating in theaxial direction with respect to the outer ring 107 a.

At the other end portion of the first accommodation section 105 a, inview of the receding orifice 157 for inserting the shaft section 103 cand the inner surface of the receiving orifice 157, a cylindricalaccommodation hole 158 drilled in the radial direction of the shaftsection 103 c, in other words, drilled in the direction of pushing theworm 103 toward the worm wheel 104 is provided instead of the supporthole 155. A bearing member 124 for rotatably joining with the shaftsection 103 c, an elastic ring 128 composed of a coil spring forapplying a force in the direction pushing the bearing member 124, and ahole closing member 129 for closing the opening of the accommodationhole 158 leading to the outside are accommodated in the accommodationhole 158. The hole closing member 129 is threaded into the opening ofthe accommodation hole 158 leading to the outside.

In the intermediate portion in the axial direction of the bearing member124, in other words, in the intermediate portion in the direction ofmovement along the accommodation hole 158, there are provided a bearinghole 124 a drilled so as to be perpendicular to the movement directionand a sliding bearing 130 inserted into the bearing hole 124 a and fixedtherein, and the shaft section 103 c is supported, so that it can movein the axial direction, by the bearing member 124 via the slidingbearing 130.

When the worm 103 moves axially in one direction (rightward) in thesteering region without driving from the motor 101, the tooth section103 a of the worm 103 applies pressure to the elastic ring 126, and theworm 103 moves rightward together with the inner ring 107 b, whilebending the elastic ring 126, and inhibits the relative movement of theinner ring 107 b and outer ring 107 a in the axial direction. Themovement quantity of the worm 103 decreases as the bending quantity ofthe elastic ring 126 increases. Furthermore, when the worm 103 movesaxially in the other direction (leftward), the locking ring 127 appliespressure to the inner ring 107 b and the worm 103 moves leftwardtogether with the inner ring 107 b, while bending the elastic ring 125,and inhibits the relative movement of the inner ring 107 b and outerring 107 a in the axial direction. The movement quantity of the worm 103decreases as the bending quantity of the elastic ring 125 increases.

Further, because the relative movement of the inner ring 107 b and outerring 107 a in the axial direction is inhibited, when the worm 103 andbearing 107 are assembled, the position of the inner ring 107 b in theaxial direction relative to outer ring 107 a and the position of theworm 103 in the axial direction relative to the bearing 107 can beeasily set and the assemblability can be improved.

Referring to FIG. 13, in the power assist unit shown in this figure, theworm 103 is supported by the bearing 107 on the motor side and by thebearing member 124 on the opposite side from the motor. In such aconfiguration, elastic rings 131, 132 serving as inhibition means forallowing the outer ring 107 a to move in the axial direction, preventingthe relative movement of the inner ring 107 b and worm 103 in the axialdirection, and inhibiting the relative axial movement of the inner ring107 b and outer ring 107 a are provided on both ends sides of the outerring 107 a.

The radii of the rolling body 107 c and the racetrack groove 107 d ofthe inner ring 107 b are the standard values, the racetrack groove 107 eof the outer ring 107 a is formed similarly to the above-mentionedracetrack groove 107 d of the inner ring 107 b, the value of the axialinner gap of the bearing 107 is larger than the value specified by theJapanese Industrial Standard, and the worm 103 can be moved in the axialdirection with respect to the outer ring 107 a and housing 105.

The outer ring 107 a is fit so that it can move in the axial directioninto the support hole 151. Elastic rings 131, 132 such as plate springs,elastic washers, or rubber sheets are provided between the outer ring107 a and control section 154 and between the outer ring 107 a and athreaded ring 111. The elastic rings 131, 132 displace the outer ring107 a toward the center of the inner ring 107 b in the axial directionand prevent the outer ring 107 a from floating in the axial directionwith respect to the inner ring 107 b.

A locking ring 133 for controlling the movement of the inner ring 107 bin the axial direction is provided in the intermediate portion of theshaft section 103 b.

When the worm 103 moves axially in one direction (rightward) in thesteering region without driving from the motor 101, the tooth section103 a of the worm 103 applies pressure to the inner ring 107 b, theinner ring 107 b moves together with the worm 103, applies pressure tothe outer ring 107 a via the inner ring 107 b and a ball 107 c, and theworm 103 moves further rightward, while bending the elastic ring 118,and inhibits the relative movement of the inner ring 107 b and outerring 107 a in the axial direction. The movement quantity of the worm 103decreases as the bending quantity of the elastic ring 118 increases.Furthermore, when the worm 103 moves axially in the other direction(leftward), the locking ring 133 applies pressure to the inner ring 107b, the inner ring 107 b moves together the worm 103 and applies pressureto the outer ring 107 a via the inner ring 107 b and rolling body 107 c,and the worm 103 moves further leftward, while bending the elastic ring132, and inhibits the relative movement of the inner ring 107 b andouter ring 107 a in the axial direction. The movement quantity of theworm 103 decreases as the bending quantity of the elastic ring 132increases.

Further, because the relative movement of the inner ring 107 b and outerring 107 a in the axial direction is inhibited, when the worm 103 andbearing 107 are assembled, the position of the inner ring 107 b in theaxial direction relative to outer ring 107 a and the position of theworm 103 in the axial direction relative to the bearing 107 can beeasily set and the assemblability can be improved.

Further, the racetrack grooves 107 d, 108 d of the inner rings 107 b,108 b also may be configured to have a straight, non-archlike surface,rather than a circular arch surface, in the central portion in the widthdirection of the racetrack grooves 107 d, 108 d. Furthermore, in theembodiment of FIG. 12, the racetrack grooves 107 e, 108 e of the outerrings 107 a, 108 a also may be configured to have a straight, nonarchlike surface, rather than a circular arch surface, in the centralportion in the width direction of the racetrack grooves 107 e, 108 e.

Further, the value of the axial inner gap of the bearings 107, 108 maybe also increased by changing the shape of the racetrack grooves of theinner rings and outer rings, rather than by changing the inner rings 107b, 108 b or racetrack grooves 107 d, 108 d of the inner ring 107 b, oras in the embodiment of FIG. 12, by changing the outer rings 107 a, 108a or the racetrack grooves 107 e, 108 d of the outer ring 107 a.

The electric power steering apparatus shown in FIGS. 6 to 13 comprises asmall gear rotated by a motor and supported by bearings and a large gearengaged with the small gear and connected to driving means and steeringassist is provided by the rotation of the motor, wherein the value ofthe axial inner gap in the ball bearings is larger than the standardvalue of the Japanese Industrial Standard (also referred to as JIS).

Because the value of the axial inner gap of the bearings supporting thesmall gear is thus set larger than the standard value, the small gearcan be further moved in the axial direction, when compared with theproducts conforming to the JIS. Moreover, the movement quantity of thesmall gear in the axial direction can be increased with respect to thatin the case where the bearings are used that have the value of the axialinner gap equal to the standard JIS value. Therefore, the steering loadin the steering region without the drive from the motor can be reducedand steering feel can be improved. Moreover, because the configurationuses no special additional mechanisms, the structure can be simplifiedand the small gear components can be miniaturized, despite the fact thatthe steering load in the steering region without the drive from themotor can be reduced.

Moreover, the above-described electric power steering apparatuscomprises inhibition means for inhibiting the relative movement of theinner rings and outer rings of the bearings in the axial direction. Whenthe relative movement of the inner rings and outer rings in the axialdirection is thus inhibited, and the small gear and bearings areassembled, the axial position of the inner ring with respect to theouter ring and the axial position of the small gear with respect to thebearing can be easily set and the assemblability can be improved.

Moreover, in the above-described electric power steering apparatus, theinhibition means is an elastic ring. Because the inhibition means can bethus constituted by inserting the elastic ring on the periphery of thesmall gear, the assemblability can be further improved.

The present invention can be used in devices for providing a steeringassist power to a wheel steering mechanism in electric power steeringdevices of automobiles.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not as restrictive. The scope of the invention is, therefore,indicated by the appended claims and their combination in whole or inpart rather than by the foregoing description. All changes that comewithin the meaning and range of equivalency of the claims are to beembraced within their scope.

1-8. (canceled)
 9. A device for supporting a worm shaft in a housing,said housing including a drive source, said worm shaft including firstand second opposing ends, said first end connecting to said drivesource, said device comprising: a first ball bearing being a deep-grooveball bearing, said bearing including an inner ring, an outer ring, and abearing ball, said inner ring and said outer ring each including aracetrack groove, said first bearing connecting said first end of saidworm shaft to said drive source wherein: said inner ring racetrack groveincludes a curvature radius of between 52.5% and 75% of the diameter ofsaid ball; or said outer ring racetrack grove includes a curvatureradius of between 53.5% and 85% of the diameter of said ball.
 10. Theworm support device of claim 9, wherein said housing includes an innerperipheral surface, and wherein: said inner ring of said first ballbearing tightly fits against said worm shaft; said device including asecond ball bearing, said second ball bearing supporting said second endof said worm shaft; and said second bearing fitting against said secondend of said worm shaft so that said bearing moves relative to one ofsaid second end of said worm shaft or said inner peripheral surface ofsaid housing.
 11. The worm support device of claim 10, wherein saidfirst bearing includes a negative gap.
 12. A power assist unit forproviding steering assist power to a wheel steering mechanism, thesteering mechanism including a steering shaft, said power assist unitcomprising: a motor including an output shaft, said motor providingrotational motion; and a worm gear mechanism for reducing the rotationpower and outputting the rotation power as said steering assist power;said worm gear mechanism further comprising: a housing connecting tosaid motor; a worm disposed within said housing, said worm includingfirst and second shaft ends, said first shaft end connecting to theoutput shaft of said motor, said worm including a gear section; a wormwheel engaged with said worm gear section, said worm wheel beingconnected to the steering shaft; first and second bearings disposed insaid housing, said first bearing supporting said first shaft end of saidworm and said second bearing supporting said second shaft end of saidworm; and said first bearing being a deep-groove ball bearing, saidbearing including an inner ring, an outer ring, and a bearing ball, saidinner ring and said outer ring each including a racetrack groove,wherein: said inner ring racetrack grove includes a curvature radius ofbetween 52.5% and 75% of the diameter of said ball; or said outer ringracetrack grove includes a curvature radius of between 53.5% and 85% ofthe diameter of said ball.
 13. The power assist unit of claim 12,wherein said housing includes an inner peripheral surface, and wherein:said inner ring of said first ball bearing tightly fits against saidworm shaft; and said second bearing fitting against said second end ofsaid worm shaft so that said bearing moves relative to one of saidsecond end of said worm shaft or said inner peripheral surface of saidhousing.
 14. The power assist unit of claim 13, wherein: said innerperipheral surface of said housing includes a large-diameter sectionsurrounding said first shaft end of said worm and a small-diametersection surrounding said second shaft end of said worm, said large andsmall diameter sections being separated by a step in said innerperipheral surface of said housing; said power assist unit furthercomprising a threaded lid, said threaded lid being threaded into thelarge-diameter section of said housing; and said outer ring of saidfirst bearing being loosely fit against said large-diameter section ofsaid housing, and said outer ring being axially disposed between saidstep in said inner peripheral surface and said threaded lid.
 15. Thepower assist unit of claim 12, wherein the outer ring of said secondbearing being press fit into said small-diameter section, said outerring being axially disposed therein and the inner ring of the secondbearing is joined by loose fitting into the shaft portion of said worm.16. The power assist unit of claim 12, wherein: the raceway groove ofthe inner ring of said first bearing includes a composite curve surface,said composite curve defining opposing shoulder regions and a bottomregion, each region including a curvature radius; and said curvatureradius of said shoulder regions being smaller than said curvature radiusof said bottom region.